1. Field of the Invention
The present invention relates to methods for fabricating a CMOS image sensor, and more particularly, to back-end-of-line methods for fabricating a CMOS image sensor.
2. Description of the Prior Art
As the development of electronic products such as digital cameras and scanners progresses, the demand for image sensors increases accordingly. In general, image sensors in common usage nowadays are divided into two main categories: charge coupled device (CCD) sensors and CMOS image sensors (CIS). Primarily, CMOS image sensors have certain advantages of low operating voltage, low power consumption, and an ability for random access. Furthermore, CMOS image sensors are currently capable of integration with the semiconductor fabrication process. Based on those benefits, the application of CMOS image sensors has increased significantly.
The CMOS image sensor separates incident light into a combination of light of different wavelengths. For example, the CMOS image sensor can consider incident light as a combination of red, blue, and green light. The light of different wavelengths is received by respective optically sensitive elements such as photodiodes and is subsequently transformed into digital signals of different intensities. Thus, it can be seen that a monochromatic color filter array (CFA) must be set above every optical sensor element for separating the incident light.
Please refer to FIGS. 1-5, which are schematic drawings illustrating a conventional method for fabricating a CMOS image sensor. As shown in FIG. 1, a semiconductor substrate 12 having a sensor array region 14 and a peripheral region 16 defined thereon is provided. A plurality of photodiodes 18 for receiving incident light, a plurality of shallow trench isolations (STIs) 20 positioned in between each photodiode 18, and a plurality of CMOS transistors (not shown) are formed in the semiconductor substrate 12. Then, at least a dielectric layer 22 is formed on the semiconductor substrate 12. Next, a plurality of patterned metal layers 24 for forming metal interconnects of the CMOS image sensor and for shielding scattering light are formed on the dielectric layer 22. Simultaneously, another patterned metal layer is formed on the dielectric layer 22 of the peripheral region 16 for serving as a contact pad 26 which provides electrical connection with external devices.
Please refer to FIGS. 1-2. Then, another dielectric layer 28 covering the patterned metal layer 24 and the contact pad 26 is formed on the dielectric layer 22, and a patterned photoresist (not shown) is subsequently formed on the dielectric layer 28. The patterned photoresist is used as an etching mask in an etching process to remove a portion of the dielectric layer 28 above the contact pad 26, thus an opening 30 exposing the contact pad 26 is formed as shown in FIG. 2.
Please refer to FIGS. 3-4. Following the formation of the opening 30, a cap layer 32 comprising an opaque metal material such as titanium or titanium nitride is formed on the dielectric layer 28. The cap layer 32 covers the contact pad 26. As shown in FIG. 3, a photoresist layer is formed on the cap layer 32, and the semiconductor substrate 12 is aligned with a photomask with the opening 30 serving as an alignment mark in an alignment step. Then, conventional exposure and development processes are performed to obtain a patterned photoresist 42. The patterned photoresist 42 serves as an etching mask in another etching process to remove the cap layer 32 above the contact pad 26 and above portions of the dielectric layer 28, thus an optical shielding layer 34 is formed on the dielectric layer 28 of the peripheral region 16 adjacent to the sensor array region 14 as shown in FIG. 4. The optical shielding layer 34 preferably serves to block external lights projecting from the peripheral region 16 to the sensor array region 14.
Please refer to FIG. 5. A plurality of color filters 36 are formed on the dielectric layer 28 of the sensor array region 14. The color filters 36 comprise red color filters, green color filters, blue color filters, or filters of other colors. Each color filter 36 corresponds to each of the photodiodes 18. Steps of forming the color filters 36 are summarized as follows: a green color filter layer (not shown) comprising photosensitive material is formed on the dielectric layer 28 by a spin coating method. Then a photomask containing green color filter pattern is provided and exposure and development processes are performed to transfer the pattern to the green color filter layer and obtain a plurality of green color filters. The same steps are used to form the blue color filters and the red color filters such that the formation of the color filters 36 is completed.
Please refer to FIG. 5 again. Thereafter, a planarizing layer 38 is formed on the color filters 36 and the optical shielding layer 34. And a polymer layer (not shown) composed of acrylate material is disposed on the planarizing layer 38. Next, a series of exposure, development, and baking processes are performed to form a plurality of micro-lenses 40 corresponding to each of the color filters 36, thus the back-end-of-line fabrication of the a CMOS image sensor 10 is completed.
As mentioned above, the color filter layer is formed on the dielectric layer 38 by spin coating step after forming the opening 30 which exposes the contact pad 26. It is noteworthy that the existence of the opening 30 makes the color filter layer store up in the opening 30 during the spin coating step, therefore the uniformity of a thickness of the color filter layer is adversely affected, and striation is caused. Striation is disadvantageous to the pixel performance, for example, saturation of the CMOS image sensor 10 is affected.